Many different configurations of internal combustion engines have been introduced historically, with inline and V-configurations having become dominant. Other configurations use opposed pistons where two pistons come together in a single combustion chamber. More recently, configurations where the combustion chamber is a toroid and where multiple pistons move in an oscillating manner have been proposed. One such engine is disclosed in U.S. patent application Ser. No. 13/074,510, filed on Mar. 29, 2011, and incorporated herein by reference. These engines have the advantage of having a high power-to-weight ratio and they offer high torque at low engine speed. These have the disadvantage, however, of being difficult to manufacture and of having unreliable piston seals due to the motion of the piston in an arc. An engine design that solves these problems is desirable.